It has long been known that one metal can be deposited upon the surface of a second metal substrate to give the second metal many of the surface characteristics of the depositing metal. Usually deposited by electrodeposition from ionic aqueous solutions of salts of the depositing metal, such coatings generally are adhered to the surface of the substrate with little if any penetration of the coating metal into the substrate metal or vice versa.
Often a surface coating having attributes such as corrosion resistance or surface hardness associated with alloys of either the substrate and the depositing metal or of two metals to be deposited upon the substrate may be desirable. One difficulty in depositing alloys by simultaneous electrodeposition of two metals from aqueous solution lies in controlling the stoichiometry of the resulting alloy due to differing activities of the metals being deposited.
Recent developments have established electrodeposition from generally anhydrous, molten salt baths as a suitable method for forming on a metal substrate an alloy surface of the substrate metal and at least one additional metal. By these methods, the metal substrate is immersed in a generally anhydrous, molten alkali or alkaline metal halide electrolysis bath containing a halide salt of the metal to be deposited and an electrode including the metal to be deposited. A controlled, generally quite low electrical current per unit surface area of the substrate is passed between the substrate and the electrode with the electrolysis bath at greater than 500.degree. C. and generally greater than 700.degree. C. The electrical current flows at generally a sufficiently low rate so that at least a considerable portion of depositing metal interdiffuses with the substrate metal at the surface of the substrate. This deposition process has come to be known as metalliding and is shown and described in U.S. Pat. No. 3,479,159 and is discussed at 221 Scientific American 38 (1969).
It is generally recognized in metalliding that the molten salt electrolysis bath should be free of carbon, its derivatives, oxygen and metal oxides.
Formation of metal films by electrodeposition from aqueous solutions including the metal upon a substrate from which those films may later be removed for further use is known. Attempts to deposit alloy films from aqueous electrolysis baths have met with disappointing results, principally due to difficulties in controlling stoichiometry of the resulting alloy due to differing activation of the metals being electrodeposited. Where substantial interdiffusion occurs between depositing metal and the substrate, resulting films are generally quite difficult to separate from the substrate in a condition for further use.